1. Field of the Invention
This invention relates to a method and apparatus for controlled mixing of two or more fluids. More particularly, this invention relates to a method and apparatus for controlled mixing of two or more fluids in which a second fluid stream is introduced into a first fluid stream in a manner which provides rapid and intimate mixing of the fluids or, alternatively, selective mixing of the fluids.
2. Description of Prior Art
There exist many applications which require rapid and intimate mixing of two or more fluids. This is generally accomplished by one or more of the following techniques: (1) using the kinetic energy of the fluids to force the mixing; (2) use of a static mixer installed in the mixture flow stream; and (3) use of a dynamic mixer installed in the mixture flow stream. A wide variety of both static and dynamic types of mixtures are available on the market. Kinetic mixing is generally accomplished by injecting a continuous lower flow rate fluid into a continuous larger flow rate fluid through strategically placed nozzles.
In many applications, for example when mixing a solid, liquid or gaseous fuel and an oxidant prior to combustion, or when mixing fuels and/or oxidants with products of partial or complete combustion, static and dynamic mixers are impractical. In the case of fuel/oxidant mixing prior to combustion, the use of static and dynamic mixers increases the potential for flashback as well as its severity, a highly undesirable consequence. In the case of mixing fuel and/or oxidant with the combustion products to change the combustion stoichiometry and/or to complete combustion, the use of static or dynamic mixers is also impractical because this requires placement of a relatively large component, namely the in-line mixer, into the combustion products.
Most combustion applications require mixing of two fluids with substantially different flow rates as well as other properties. For example, in combustion applications employing premixed fuel and oxidant, the fuel, for example, natural gas, must be mixed with oxidant, for example, air, in proportions of about 1:10 in order to obtain stoichiometric combustion. Similarly, for applications requiring the injection of a fuel into the combustion products, such as gas reburning, or applications that require injection of an oxidant into the combustion products, for example, secondary and/or tertiary combustion air in staged combustion, the injected fluid is only a small fraction of the total volume of the combustion products. In all of these applications, difficulty arises from the need to simultaneously provide both distribution and penetration of the injected fluids so as to provide rapid and uniform mixing. One solution to the problem of disparate volumes and flow rates of an injected fluid and a fluid being injected into is the use of carrier fluids, that is, the mixing of the injected fluid with an additional fluid (the carrier fluid) prior to injection. In the case of injection of a fuel into a stream of combustion products, for example, nitrogen or steam or recirculated flue gases may be employed as a carrier fluid. This, of course, requires additional equipment by the user.
Other applications require selective mixing of fluids. For example, if a first fluid, such as combustion products, has a non-uniform (or unsteady) oxygen profile which needs to be made uniform (or steady) by mixing it with a fluid containing a fixed level of oxygen, then the most effective way to achieve the uniform profile of oxygen in the combustion product is to selectively inject the oxygen-containing fluid into the combustion product, that is, injecting more oxygen-containing fluid where or when the oxygen concentration in the combustion product is lower than desired and less oxygen-containing fluid where or when the oxygen concentration in the combustion products is higher than desired. Similarly, in some applications, controlled mixing may be necessary to provide a certain profile of a particular parameter, for example, species concentration, temperature, etc., in the resulting mixture. Conventional mixers can be designed to provide a certain level of mixing for a fixed ratio of flow of two streams, but the adequacy of the mixing decreases as the ratio is varied. U.S. Pat. No. 4,779,545 teaches a method and apparatus for reducing NO.sub.x, emissions from furnace flue gas in which natural gas or another fluid having no fixed nitrogen is introduced in pulses, that is discontinuous injections, into the upper portion of the furnace so as to obtain a desired form of mixing.
Related U.S. Pat. Nos. 4,824,557; 4,808,382; 4,711,766; and 4,624,836 all relate to a plug valve for providing immediate and intimate mixing of fluidizable cracking catalysts with fluid hydrocarbons in a riser reactor. The plug valve comprises a generally hollow tubular plug stem having a hollow plug secured thereto. The plug is engageable with the riser reactor access opening and movable in a reciprocating fashion to permit and control the flow of fluidizable cracking catalysts to the riser reactor. U.S. Pat. No. 4,521,117 teaches an arrangement for mixing a first gas into a main flow of a second gas comprising a main conduit for the main flow and a plurality of supply conduits for the first gas which open into the main conduit. The supply conduits are arranged as at least one set of three outlet openings, the first opening of which opens into the main flow perpendicular to the main conduit wall, and the remaining openings of which are arranged so as to provide a lesser flow rate of the first gas than the first opening and are offset with respect to the first opening. The three openings of the set thus direct the first gas into two contra-rotating circulatory movements as viewed in the direction of the axis of the main conduit.
An apparatus for mixing one or more gases into a main flow of gas in a cylindrical conduit through one or more feed conduits is taught by U.S. Pat. No. 4,390,346. The feed conduits are provided with two openings for outflow of the added gas, which openings are spaced from the wall of the cylindrical conduit, are symmetrically located with respect to the axis of the cylindrical conduit, and lie in a plane perpendicular to that axis. The axes of these openings are mutually spaced by less than one-half of the internal diameter of the conduit. The added gas emerges from the two openings in mutually opposite parallel directions so that it tends to circulate around the axis.
Finally, U.S. Pat. No. 3,734,111 teaches a method and apparatus for mixing a fluid traveling in a pipe section with a second fluid introduced into the pipe section by sparging the second fluid into the pipe section at points across the internal diameter of the pipe section, disrupting the flow through the pipe section with a perforated frusto-conical baffle coaxially aligned with the pipe section with its apex extending toward the means of sparging the second fluid, sparging additional second fluid downstream from within the flow of the frusto-conical baffle, and disrupting the flow within the pipe downstream of the frust-conical baffle and sparging means with a line restriction extending from the pipe section sidewalls into the pipe section to create an orifice of smaller diameter than the pipe section inside diameter coaxially aligned with the pipe sections. None of these methods are suitable for selective mixing of fluids, nor are they particularly suitable for applications requiring mixing of two fluids of substantially different flow rates.